The present invention relates in general to printed wiring boards (PWBs) and other substrates, and to design structures for same. More particularly, the present invention relates to tuning the propagation velocity of a signal through a conductive trace of a PWB or other substrate by interposing a permittivity attenuation layer containing functionalized carbon nanomaterial, such as functionalized single-wall carbon nanotubes (f-SWNTs), between the conductive trace and the substrate, and to a design structure for same.